Swollen knee

A swollen knee occurs when excess fluid accumulates in or around your knee joint. Your doctor might refer to this condition as an effusion (ih-FYU-zhen) in your knee joint. Some people call this condition "water on the knee."

A swollen knee may be the result of trauma, overuse injuries, or an underlying disease or condition. To determine the cause of the swelling, your doctor might need to obtain a sample of the fluid to test for infection, disease or injury.

Removing some of the fluid also helps reduce the pain and stiffness associated with the swelling. Once your doctor determines the underlying cause of your swollen knee, appropriate treatment can begin.

Torn meniscus

ACL injury

Age. Your likelihood of developing a swollen knee related to arthritis increases as you age.

Sports. People who participate in sports that involve twisting the knee, such as basketball, are more likely to experience the types of knee injuries that cause swelling.

Obesity. Excess weight puts added stress on the knee joint, contributing to the tissue and joint overload and knee degeneration that can lead to a swollen knee. Obesity increases your risk of osteoarthritis, one of the more frequent causes of knee swelling.

Muscle loss. Fluid in the knee can harm the working of your muscles and cause thigh muscles to weaken and atrophy.

Fluid-filled sac (Baker's cyst). The buildup of fluid in your knee can lead to the formation of a Baker's cyst in the back of your knee. A swollen Baker's cyst can be painful, but usually improves with icing and compression. If the swelling is severe, you might need to have fluid removed (cyst aspiration).

Ice and elevation. To control pain and swelling, apply ice to your knee for 15 to 20 minutes every two to four hours. When you ice your knee, raise your knee higher than the level of your heart, using pillows for comfort.